1. Field of the Invention
The present invention relates to a lead frame retaining device for use in a wire- or die-bonding apparatus.
2. Prior Art
Conventionally, two types of lead frame retaining assemblies are known.
One type has a bonding window opened in a lead frame retaining member. This type of retaining assembly is disclosed in, for example, Japanese Patent Application Publication Nos. 51-21744 and 51-44865 and also in Japanese Utility Model Application Publication No. 2-9557 (called "Conventional Example 1").
Another type of retaining assembly has also a bonding window in a lead frame retaining member and further includes a splitted lead retaining member inside the bonding window so that each lead of a lead frame is retained by slits formed by the splitted lead retaining member. This type is disclosed in, for instance, Japanese Utility Model Application Publication No. 59-5976 (called "Conventional Example 2").
In either the conventional Example 1 or 2, the lead frame retaining member and a heating block, that heats lead frames, are provided on respective blocks which are independently driven up and down.
In Conventional Example 1, the leads of lead frame are retained by a flat surface of the lead retaining section of the lead frame retaining member. As a result, it has several problems.
If the flat surface of the lead retaining section is not completely parallel to the upper surface of the heating block, portions of the lead retaining section will contact the leads in a skewed manner, and some of the leads stand up. If wire bonding is performed under this condition, faulty bonding will occur since the thermal conductivity is poor. In addition, every time the type of lead frame being handled is changed, the lead frame retaining member also needs to be changed so as to meet the new lead frame; and if the retaining member is changed, a high-precision alignment of the parts is required. This work is not easy, and it is difficult to obtain a perfect alignment between the lead retaining section and heating block.
In view of this inconvenience, Conventional Example 2 uses a system in which the lead retaining section is splitted by slits so that the retaining section corresponds to the respective leads of the lead frame. Accordingly, even if the undersurface of the lead retaining section is not completely parallel to the upper surface of the heating block, or even if there is some warping, etc. in the leads, the lead retaining section can bend elastically so that all the leads are properly retained. In this respect, Conventional Example 2 is better than Conventional Example 1.
In actuality, however, it is difficult to securely retain all the leads by the lead retaining section. More specifically, in order to retain all the leads securely, the lead retaining section must have an appropriate rigidity and elasticity. Nevertheless, the rigidity and elasticity are completely opposite properties. Thus, it is difficult to manufacture a lead frame retaining member which has both an appropriate rigidity and an appropriate elasticity. If the rigidity is increased, the elasticity suffers, causing the same problems as seen in Conventional Example 1. Conversely, if the elasticity is increased, the rigidity drops and warping of the leads and error between the heating block and the lead retaining section cannot be completely absorbed. Furthermore, a long-term use can easily cause elasticity deteriorates, etc.